dax : Fix documentation with respect to struct pages

mapped caches such as ARM, MIPS and SPARC.

mapped caches such as ARM, MIPS and SPARC.

Calling get_user_pages() on a range of user memory that has been mmaped

Calling get_user_pages() on a range of user memory that has been mmaped

from a DAX file will fail as there are no 'struct page' to describe

from a DAX file will fail when there are no 'struct page' to describe

those pages.  This problem is being worked on.  That means that O_DIRECT

those pages.  This problem has been addressed in some device drivers

reads/writes to those memory ranges from a non-DAX file will fail (note

by adding optional struct page support for pages under the control of

that O_DIRECT reads/writes _of a DAX file_ do work, it is the memory

the driver (see CONFIG_NVDIMM_PFN in drivers/nvdimm for an example of

that is being accessed that is key here).  Other things that will not

how to do this). In the non struct page cases O_DIRECT reads/writes to

work include RDMA, sendfile() and splice().

those memory ranges from a non-DAX file will fail (note that O_DIRECT

reads/writes _of a DAX file_ do work, it is the memory that is being

accessed that is key here).  Other things that will not work in the

non struct page case include RDMA, sendfile() and splice().